Without using the website, I suspect you can use something like Fiddler to see what HTTP requests are being made. Chances are the charts are making an XML or JSON request to their servers.

If you're lucky, the URL requests will be in an easy to reverse engineer format so you could easily dump all the data you need by adjusting the URL.

Why not try to intercept the data as it comes through the USB port instead?

Or try liftbit as noted above: https://github.com/openyou/libfitbit

libfitbit unfortunately doesn't currently work with the fitbit one, for the older devices it's apparently fine but the one changes how it gets talked to entirely. I've attempted a bit of work with getting something talking to it but it's not an easy thing to do. Their bluetooth dongle presents itself as an HID device and I think hides a lot of the details away from you if you wanted to talk to it over actual bluetooth. I'd love to get it to sync with both their site and to be able to capture the information myself to store.

Looks like they are rendering the charts out via JSON data end points - should be pretty easy to grab the data that way.

The flash content must get the data from somewhere, probably in XML, certainly over HTTP. Open Wireshark and capture a session to known where it came from. Then write a program that replicates this behavior and save the data.

I just ran it through fiddler and the Flash content gets the details in XML over HTTP. For steps and floors you get a count in 5 minute units; for calories you get a burn in 5 minute units when you have been active; for sleep you seem to get a score to indicate whether you are asleep or awake every minute you are asleep.